# I Missing exponent in "Theoretical Minimum"?

#### SamRoss

Gold Member
Summary
Is there a missing exponent in the authors' application of the Euler-Lagrange equation?
In "The Theoretical Minimum" (the one on classical mechanics), on page 218, the authors write a Lagrangian

$$L=\frac m 2 (\dot r^2 +r^2\dot \theta ^2)+\frac {GMm} r$$

They then apply the Euler-Lagrange equation $\frac d {dt}\frac {dL} {d\dot r}=\frac {dL} {dr}$ (I know there should be partial derivatives there but I couldn't find the symbol for it in my Latex primer; if anyone could enlighten me, I'd appreciate it) and wrote the result...

$$\ddot r=r\dot \theta^2-\frac {GM} r$$

My question is, shouldn't the last r in the denominator be squared since it results from differentiating the GMm/r term in the Lagrangian by r?

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#### vanhees71

Gold Member
Indeed, there should be an $r^2$ in the denominator.

#### Ibix

...and \partial gives you $\partial$.

#### SamRoss

Gold Member
Indeed, there should be an $r^2$ in the denominator.
Thanks, I thought so. The original version of the book apparently lacked both exponents in the result. The online errata for the book shows that, as a correction, the first exponent (above theta dot) was put in but the second exponent is still missing. Weird. However, on the next page of the book the entire equation is written out again, correctly this time. I hadn't moved on to that page before making this post.

#### vanhees71

Gold Member
Well unfortunately typos are very persistent beasts. That's why you havd to carefully check all the formulae yourself in every writing, whereever published!

"Missing exponent in "Theoretical Minimum"?"

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